1. Field of the Invention
This invention is related to the food and pharmaceutical industry and is particularly concerned with an apparatus for continuous microwave and vacuum drying, in which (i) glow discharge is prevented and (ii) the material to be treated is submitted to foaming at a low temperature (for instance at a temperature lower than or equal to 40.degree. C., if required), and which is useful for obtaining a dried end-product having excellent rehydration capabilities.
2. Description of the Prior Art
The principle of using microwaves and vacuum for drying is known. U.S. Pat. No. 3,276,138 describes a certain number of possibilities, the object of all of which is to prevent ionisation, but the device of this patent, like those described in U.S. Pat. Nos. 2,859,534 and 3,048,928, relate to apparatuses which function intermittently.
In these apparatuses, the electric field increases as the amount of energy abosrbed by the quantity of product present decreases, this decrease being due to evaporation and removal of the liquid as heating progresses.
The critical value of the field is thus reached quickly, and to prevent the ionisation which would result therefrom the microwave power must be reduced, which leads to a decrease in the rate of drying and to use of the apparatus below its maximum yield.
Moreover U.S. Pat. No. 3,365,562 discloses a microwave oven for cooking foodstuffs, which comprises a heating compartment which is not hermetically closed, and reflectors which are provided on the top side of the compartment where microwaves are injected. U.S. Pat. No. 3,507,050 proposes a meander waveguide for drying sheet materials at atmospheric pressure and use of rollers outside the microwave field to convey the sheets. U.S. Pat. No. 3,545,093 discloses a means for microwave removal of volatile polar vehicles from non-polar materials, and especially a resonating cavity with air inlets and outlets and a vibrating plate as a material handling means, but it does not teach how to affect microwave evaporation under vacuum in a continuous apparatus. U.S. Pat. No. 3,780,251 describes a tunable waveguide the dimensions of which do not change. U.S. Pat. No. 3,834,038 discloses an apparatus for drying plastic resins with microwave energy in a partial vacuum, which does not comprise a primary non-evacuated microwave cavity through which microwaves are fed into an evacuated chamber as it is the case in this application.